Core Issue 1512: Pointer comparison vs qualification conversions

Introduction

This paper presents the modifications to the Working Draft necessary
to resolve core issues 583 and 1512. In particular, it makes

void f(char * p)
{
if (p > 0) { ... }
if (p > nullptr) { ... }
}

ill-formed (both cases) and

void g(int **p1, const int**p2)
{
if (p1 == p2) { ... }
}

well-formed.

Explanation of the changes

The changes below essentially replace all of 5.9 expr.rel and 5.10
expr.eq. The current description in the Working Paper mixes semantic
constraints and results of relational operators with those of equality
operators. Furthermore, the attempt at type unification for similar
types fails spectacularly.

The changes below define a generic composite pointer type that is
applicable to relational, equality, and conditional operators. The
corresponding description is moved from section 5.9 expr.rel to clause
5 expr.

Furthermore, the semantic specifications of the relational and equality
operators are now phrased in terms of "compares greater" and "compares
equal"; the result of the operators is derived from these
specifications. This avoids repetition and makes it possible to refer
to 5.10 expr.eq in order to determine the results of the <= and
>= operators for some cases.

For the equality operators, 5.10 expr.eq no longer refers to 5.9
expr.rel. Instead, the relevant semantic constraints are specified
again (grouping, permissible types, result type). Each paragraph then
enumerates one of the cases of types of operands (pointer, pointer to
member, std::nullptr_t, arithmetic or enumeration type).

The special wording about union members was phrased in terms of "same
address" and moved to 9.5 class.union, where layout of union members
is discussed.

Finally, the overload descriptions for built-in operators were
adjusted, because objects of type std::nullptr_t cannot
be used with relational operators any more.

Changes to the Working Paper

Add a new paragraph at the end of 5 expr:

The cv-compatible type of two types T1 and T2 is a type T3
similar to T1 whose cv-qualification signature (4.4 conv.qual) is:

for every j > 0, cv3,j is the union of
cv1,j and cv2,j

if the resulting cv3,j is different from
cv1,j or cv2,j, then
const is added to every cv3,k for 0
< k < j

[ Note: This construction ensures that both T1 and T2 can be converted
to T3. ] The composite pointer type of two operands p1 and
p2 having types T1 and T2, respectively, is:

if both p1 and p2 are null pointer constants,
std::nullptr_t;

if either p1 or p2 is a null pointer constant, T2 or T1,
respectively;

if T1 or T2 is "pointer to cv1void" and the
other type is "pointer to cv2 T", "pointer to cv12void" where cv12 is the union of cv1
and cv2;

if T1 is "pointer to cv1 C1" and T2 is "pointer to
cv2 C2" where C1 is reference-related to C2 or C2 is
reference-related to C1 (8.5.3 dcl.init.ref), the cv-compatible type
of T1 and T2 or the cv-compatible type of T2 and T1,
respectively;

if T1 is "pointer to member of C1 of type cv1 U1" and T2 is
"pointer to member of C2 of type cv2 U2" where C1 is
reference-related to C2 or C2 is reference-related to C1 (8.5.3
dcl.init.ref), the cv-compatible type of T2 and T1 or the cv-compatible type
of T1 and T2, respectively;

if T1 and T2 are similar multi-level mixed pointer and pointer to
member types (4.4 expr.qual), the cv-compatible type of T1 and
T2;

otherwise, a program that necessitates the determination of a
composite pointer type is ill-formed.

The composite pointer type of p and q is "pointer to const void"; the
composite pointer type of pi and pci is "pointer to const pointer to
const int".
]

The following also resolves core issue 583.

Change in 5.9 expr.rel paragraphs 1 to 5:

... The operands shall have arithmetic, enumeration, or pointer
type, or type std::nullptr_t.

The usual arithmetic conversions are performed on operands of
arithmetic or enumeration type.
If both operands are pointers, pointerPointer
conversions (4.10 conv.ptr)
and qualification conversions (4.4 conv.qual) are performed
on pointer operands (or on a pointer operand and a null pointer
constant, or on two null pointer constants, at least one of which is
non-integral) to
bring them to their composite pointer typecomposite pointer type (clause 5 expr).
If one operand is a null
pointer constant, the composite pointer type is
std::nullptr_t if the other operand is also a null
pointer constant or, if the other operand is a pointer, the type of
the other operand. Otherwise, if one of the operands has type "pointer
to cv1 void," then the other has type "pointer to cv2 T " and the
composite pointer type is "pointer to cv12 void," where cv12 is the
union of cv1 and cv2. Otherwise, the composite pointer type is a
pointer type similar (4.4 conv.qual) to the type of one of the
operands, with a cv-qualification signature (4.4 conv.qual) that is
the union of the cv-qualification signatures of the operand types. [
Note: this implies that any pointer can be compared to a null pointer
constant and that any object pointer can be compared to a pointer to
(possibly cv-qualified) void. -- end note ] [ Example:

-- end example ]After conversions, the operands shall have the same type.

Pointers to objects or functions of the same type (after
pointer conversions) can be compared, with a resultComparing pointers to objects is defined as
follows:

If two pointers p and q of the same type point to the same
object or function, or both point one past the end of the same array,
or are both null, then p<=q and p>=q both yield true and p<q
and p>q both yield false.

If two pointers p and q of the same type point to
different
objects that are not members of the same object or elements of the
same array or to different functions, or if only one of them is null,
the results of p<q, p>q, p<=q, and p>=q are unspecified.

If two pointers point to elements of the same array or one
beyond the end of the array, the pointer to the object with the higher
subscript compares greater.

If two pointers point to non-static data members of the same
object, or to subobjects or array elements of
such members, recursively, the pointer to the later declared member
compares greater
provided the two members have the same access control (Clause 11) and
provided their class is not a union.

If two pointers point to non-static data members of the same
object with different access control (Clause 11) the result is
unspecified.

If two pointers point to non-static data members of the same
union object, they compare equal (after conversion to void*, if
necessary). If two pointers point to elements of the same
array or one beyond the end of the array, the pointer to the object
with the higher subscript compares higher.

Other pointer comparisons are unspecified.

If two operands p and q compare equal (5.10 expr.eq), p<=q and
p>=q both yield
true and p<q and p>q both yield
false. Otherwise, if a pointer p compares greater than a
pointer q, p>=q and p>q both yield true and p<=q
and p<q both yield false. Otherwise, the result of
each of the operators is unspecified.

Pointers to void (after pointer conversions) can be compared,
with a result defined as follows: If both pointers represent the same
address or are both the null pointer value, the result is true if the
operator is <= or >= and false otherwise; otherwise the result
is unspecified.

If two operands of type std::nullptr_t are compared, the
result is true if the operator is <= or >=, and false otherwise.

If both operands (after conversions) are of arithmetic or enumeration
type, each of the operators shall yield true if the
specified relationship is true and false if it is false.

Change in 5.10 expr.eq paragraphs 1 to 4:

The == (equal to) and the != (not equal to) operators group
left-to-right. The operands shall have arithmetic, enumeration,
pointer, or pointer to member type, or type
std::nullptr_t. The operators == and != both yield
true or false, i.e. a result of type
bool.have the same
semantic restrictions, conversions, and result type as the relational
operators except for their lower precedence and truth-value
result.
[ Note: a<b == c<d is true whenever a<b and c<d have the same
truth-value. -- end note ]In each case below, the operands shall have the same type after
the specified conversions have been applied.

If one of the operands is a pointer, pointer conversions (4.10
conv.ptr) and qualification conversions (4.4 conv.qual) are performed
on both operands to bring them to their composite pointer type (clause
5 expr). Comparing pointers is defined as follows:Pointers of the same type (after pointer conversions) can be
compared for equality. Two pointers of the same type
compare equal if
and only if they are both null, both point to the same
function, or both represent the same address (3.9.2
basic.compound), otherwise they compare unequal.

If one of the operands is a pointer to member, pointerIn addition, pointers to members can be compared, or a pointer to
member and a null pointer constant. Pointer to member
conversions (4.11 conv.mem) and qualification conversions (4.4
conv.qual) are performed on both operands to bring them to
a common typetheir composite pointer type (clause 5
expr).
If one operand is a null pointer constant, the common type is the
type of the other operand. Otherwise, the common type is a pointer to
member type similar (4.4 conv.qual) to the type of one of the
operands, with a cv-qualification signature (4.4 conv.qual) that is
the union of the cv-qualification signatures of the operand types. [
Note: this implies that any pointer to member can be compared to a
null pointer constant. -- end note ]Comparing pointers to member is defined as follows:

If both operandstwo pointers to member are
both the null member pointer value, they compare
equal.

Otherwise ifIf only one of two pointers to
member is the null member pointer value,
they compare unequal.

Otherwise ifIf either is a pointer to a
virtual member function, the result is unspecified.

Otherwise theyTwo pointers to member
compare equal if and only if they would refer to the same
member function of
the same most derived object (1.8 intro.object) or to the
same subobject
if they were dereferenced with a hypothetical object of the associated
class type, otherwise they compare unequal. [ Example:

If twoTwo operands of type
std::nullptr_t or one operand of type
std::nullptr_t and the other a null pointer
constant compare equalare
compared, the result is true if the operator is ==, and false
otherwise.

If two operands compare equal, the result is true
for operator== and false for operator!=. If two operands
compare unequal, the result is false for operator== and
true for operator!=. Otherwise, the result of each of
the operators is unspecified.

If both operands are of arithmetic or enumeration
type, the usual arithmetic conversions are performed on both
operands;
eachEach of the operators shall yield true if
the specified relationship is true and false if it is false. [
Note: a<b == c<d is true whenever a<b and c<d have the
same truth-value. -- end note ]

Change in 5.16 expr.cond paragraph 6:

...

One or both of theThe second and third
operands have pointer type, or one has
pointer type and the other is a null pointer constant, or both are
null pointer constants, at least one of which is non-integral;
pointer
conversions (4.10 conv.ptr) and qualification conversions (4.4
conv.qual) are performed to bring them to their composite pointer type
(5.9 expr.rel)(5 expr). The result is of
the composite pointer type.

One or both of theThe second and third operands have pointer to member type, or one
has pointer to member type and the other is a null pointer
constant;
pointer to member conversions (4.11 conv.mem) and qualification
conversions (4.4 conv.qual) are performed to bring them to a common
type, whose cv-qualification shall match the cv-qualification of
either the second or the third operandtheir composite
pointer type (5 expr). The result is of the
commoncomposite pointer type.

Both the second and third operand have type
std::nullptr_t or one has that type and the other is a
null pointer constant. The result is a of type
std::nullptr_t.

Change in 9.5 class.union paragraph 1:

... Each non-static data member is allocated as if it were the sole
member of a struct. All non-static data members of a union object
have the same address. ...

Change in 13.6 over.built paragraphs 15 and 16:

For every T , where T is an enumeration type,or
a pointer type, or std::nullptr_t, there exist
candidate operator functions of the form